1. Field of the Invention
The invention relates to an exhaust purification system for an internal combustion engine.
2. Description of the Related Art
The exhaust from an internal combustion engine contains harmful substances such as NOx and the like. It is known that in order to lessen the emission of such harmful substances, the exhaust system of an internal combustion engine is provided with an exhaust purification device that includes a NOx catalyst for removing NOx from exhaust.
The exhaust purification device that includes a NOx catalyst is able to remove NOx while the temperature thereof is kept at or above the activation temperature of the catalyst. Therefore, at the time of cold start of the internal combustion engine, the temperature of the exhaust purification device has not reached the activation temperature of the catalyst, so that sufficient removal of NOx from exhaust may become difficult and emissions may deteriorate. Therefore, at the time of cold start of the internal combustion engine, it is necessary to quickly raise the temperature of the exhaust purification catalyst to or above the activation temperature of the catalyst so as to allow removable of NOx from the exhaust.
In conjunction with a case where an internal combustion engine equipped with an exhaust purification device as described above has a plurality cylinder groups, a construction in which a dual-construction exhaust purification device is disposed downstream of the point of confluence of streams of exhaust from the cylinder groups is described in Japanese Patent Application Publication No. JP-A-2002-364352. In such an exhaust purification system, too, it is necessary that at the time of cold start, the temperature of the dual-construction exhaust purification device be raised quickly.
If the above-described exhaust purification device employs, for example, a three-way catalyst as a NOx catalyst, the device is able to remove HC, CO and NOx simultaneously in a nearly stoichiometric atmosphere. Furthermore, if, for example, a storage reduction type NOx catalyst is provided as a NOx catalyst, the device is able to remove NOx from exhaust due to the function of storing NOx from exhaust when the oxygen concentration in inflow exhaust is high, and of reducing stored NOx when the oxygen concentration in inflow exhaust has dropped and a reductant is present.
The NOx removal performance of the storage reduction type NOx catalyst deteriorates if the amount of NOx stored therein increases. Therefore, the storage reduction type NOx catalyst is supplied with a reductant to reduce and release NOx stored (hereinafter, referred to as “NOx reduction process”). In some other cases, in order to resolve the SOx poisoning in which SOx from exhaust is stored in the NOx catalyst and deteriorates the NOx removal performance, the catalyst temperature of the NOx catalyst is raised and a reductant is supplied thereto (hereinafter, referred to as “SOx poisoning recovery process”).
Furthermore, in the exhaust purification system including a dual-construction exhaust purification device, the storage reduction type NOx catalyst, as for example, has the functions of storing and reducing NOx from exhaust when the temperature of the catalyst is within the range of 350° C. to 450° C. Therefore, the purification efficiency of the exhaust purification device depends on the temperature of the exhaust introduced into the device. Therefore, in some cases, it is difficult to maintain high purification efficiency of the exhaust purification device in a wide range of operation states from a low-load/low-rotation speed operation state to a high-load/high rotation speed operation state.